Liquid products, particularly petroleum-based products, are often stored in very large quantities in above-ground tanks. One concern with such tanks is the possibility that the tank may develop a leak which is not readily discernable through normal methods of inventory control or visual inspection. Significant environmental concerns arise with leaks which are relatively minor when viewed over a short period of time. Such leaks, when left undetected and unrepaired, often result in very substantial leakage over a long period of time.
The problem of leak detection becomes even more acute as the size of the storage tank increases since, as may be envisioned, minor leakage from such tanks will cause only a small change in the level of the liquid in storage. This is particularly true in tanks greater than about 20 feet in diameter. Also, in above-ground tanks, changes in ambient conditions, such as temperature and wind, can affect the level of the liquid in the tank.
A common practice used in industry for assessing above-ground storage tank integrity is to empty the tank of its volatile contents, re-fill the tank with water to a level of at least six feet to provide sufficient head pressure and use visual detection of liquid level variations through a sight glass outside of the tank to measure for possible leaks. As may be readily envisioned, this technique has several disadvantages. One disadvantage is that the tank must be taken out of service periodically to assess its integrity. Another disadvantage is that the water used to conduct the test must either be disposed of as hazardous waste, as a result of its contacting the interior surfaces of the storage tank, or sent to a waste water treatment plant, a facility rarely present at most locations. However, an even more significant disadvantage involves the concern for test accuracy, since visual detection is limited to about 1/16 of an inch variation in liquid level. In a 50 foot diameter tank, 1/16 of an inch equates to a minimum detectable volume in excess of approximately 75 gallons, while in a 100 foot diameter tank, 1/16 of an inch equates to a minimum detectable volume in excess of approximately 300 gallons. Also, changes in temperature, which can cause the liquid in the tank to expand or contract, can significantly affect the measurement as can wind effects and sight glass meniscus resolution.
Another technique which has been considered is to place a pressure transducer adjacent to the bottom of the tank to measure the pressure of the liquid at the bottom of the tank. This technique is not affected by changes in temperature since, although the volume of the liquid may change due to liquid expansion, its weight does not change. This technique has the problem that it requires a pressure transducer that can not only measure the heavy weight of the liquid in the tank, but which can also measure slight variations in the weight. Such a transducer is either not readily available or is prohibitively expensive.
Swiss Patent No. 607,011 discloses a checking device for leakage from large liquid reservoirs and tanks which utilizes a pressure sensitive detector having two chambers separated by a diaphragm. One chamber of the detector is connected to the reservoir and the other to a reference tube. The diaphragm of the pressure sensitive detector can be moved toward a contact member to close an electrical signalling circuit in the event that a leak develops. However, the arrangement disclosed in Swiss Patent No. 607,011 would not be expected to have the accuracy required to assure tank integrity in light of today's environmental concerns and regulations.
Ser. No. 183,121, filed on Apr. 18, 1989, an application in which I am the inventor of the subject matter, discloses a liquid leak detector for above-ground tanks. The leak detector taught therein includes an upright standpipe connected to a tank in such a manner that the standpipe can be filled with the liquid in the tank. Also included is a pressure transducer connected between the bottom of the tank and the bottom of the standpipe to measure the difference in pressure of the liquids in the tank and standpipe. As may be envisioned, any change in the liquid pressure at the bottom of the tank resulting from leakage therefrom will result in a change in the pressure differential measured by the pressure transducer. By utilizing the preferred liquid to liquid pressure transducer disclosed in Ser. No. 183,121, such transducer having the ability to detect pressure head differences on the order of 0.001 inches of water column, good detection capabilities were demonstrated for tests conducted utilizing water and heating oil as the tank fluids. While Ser. No. 183,121 states that the leak detector taught could be used satisfactorily with very volatile liquids whose vapors are heavier than air, such as gasoline, improvements in that system were nevertheless sought to increase test result accuracy when using such volatile fluids. Ser. No. 183,121 is hereby incorporated by reference for all that it discloses.
Therefore, it would be desirable to have a liquid leak detector capable of accurately detecting small leaks in large above-ground tanks which is not adversely affected by the volatility characteristics of the product being stored or by changes in ambient conditions present during the course of assessing tank integrity.